In digital signaling, the voltage swing of the signal often encounters significant noise. This is especially the case with low voltage signals at a high speed interface. In such applications, problems such as power noise or ground noise can be the result of simultaneous switching of input/output (I/O) buffers, signal reflections, or signal cross-talk. Noise can also result from a weakened physical interconnection caused by a bad solder joint, an improper connection, etc. The result of such noise is the reduction of signal voltage margin as well as degradation of signal performance and reliability.
FIG. 1A shows a graphical depiction of an example of an ideal signal 10. The graph represents the voltage of the signal as depicted over time. VREF represents the mid-point reference of the voltage. VREF_L represents the lower voltage boundary. Any signal voltage level below this point, is considered a LOW signal value. VREF_H represents the high voltage boundary. Any signal voltage level above this point is considered a HIGH signal value.
In FIG. 1A, the signal 10 starts LOW and transitions to HIGH before returning to LOW. The signal 10 clearly and unambiguously makes the successful transition in signal value. However, FIG. 1B shows the effect on the signal 20 of noise caused by a ground glitch. The signal 20 never clearly makes the transition from LOW to HIGH. Instead, it is stuck in the transitional area between VREF_L and VREF_H. FIG. 1C shows the effect on the signal 30 of noise caused by a power glitch. The signal 30 never clearly makes the transition from HIGH to LOW. As with the ground glitch of FIG. 1B, the signal is stuck in the same transitional area without a clear, discernable value. Finally, FIG. 1D shows an example of the effect of a high resistance path. The signal 40 never crosses VREF_L during its transition from HIGH to LOW. As seen in the previous examples, its value it indeterminate.
In general, electrical characterization of noise related problems require proper software support in generating data patterns and robust hardware support to determine the transient voltage. A common method of debugging a system involves pin by pin testing to find signal voltage errors. This is a very costly and time-consuming operation whether in the laboratory, in the production facility, or in the field.